Ice storm story

[Spudford]

This story is a few months old already, but after telling it at a party this afternoon, it occurred to me that it is worth sharing.

I took delivery of my Model 3 AWD in early November, and the day after Thanksgiving, I took off on my first road trip from Pensacola. My first objective was to see how much less taxing it was to drive a long distance in the Tesla. The first day I covered 900+ miles in about 18 hours, and afterward, my eyes were the only part of me that was tired. I let Elon do a lot of the driving that day, and I was impressed with how much less tiring the day was with less noise/vibration and less mental bandwidth dedicated to driving.

The next morning, driving north from northern Virginia, it was drizzling. Once I got on the interstate, I watched the temperature drop through the 30s and below freezing. The windshield started icing over, and I got flustered because I had not used the defroster, and was not sure where to engage it. But I found it, and the blower was blasting out hot air soon, clearing the windshield. While it was blowing, I checked the energy use screen our of curiosity. It was high. I don't remember how high -- 400, 500, 600 Kw/mi -- but it startled me a little, and as soon as the ice was clear, I cut off the defroster. I checked again a little later, and the rate of energy use was still quite elevated. Why would that be? I quickly figured out how my plan for Supercharger stops would need to be adjusted to accommodate the energy use, and carried on.

It wasn't until a few days later that I realized what (I think) was the reason for the dramatically decreased range: a coat of bumpy ice over the body's surface not only added some weight, it also messed up the car's aerodynamics. The sloppy road surface contributed too, I'm sure, but it really seems to me that the drag issue must have been primary.

The rest of the trip was smooth as can be. I put over 3000 miles on the odometer in a week.

But the conclusion I took from the ice storm incident was that the Model 3 relies on the low, low drag coefficient quite a bit to achieve the range and efficiency the car has.

Okay, discuss amongst yourselves.

 

[Spudford]

The unit, of course, should be Wh/mi. [embarrassed emoticon]

 

[coleAK]

This story is a few months old already, but after telling it at a party this afternoon, it occurred to me that it is worth sharing.

I took delivery of my Model 3 AWD in early November, and the day after Thanksgiving, I took off on my first road trip from Pensacola. My first objective was to see how much less taxing it was to drive a long distance in the Tesla. The first day I covered 900+ miles in about 18 hours, and afterward, my eyes were the only part of me that was tired. I let Elon do a lot of the driving that day, and I was impressed with how much less tiring the day was with less noise/vibration and less mental bandwidth dedicated to driving.

The next morning, driving north from northern Virginia, it was drizzling. Once I got on the interstate, I watched the temperature drop through the 30s and below freezing. The windshield started icing over, and I got flustered because I had not used the defroster, and was not sure where to engage it. But I found it, and the blower was blasting out hot air soon, clearing the windshield. While it was blowing, I checked the energy use screen our of curiosity. It was high. I don't remember how high -- 400, 500, 600 Kw/mi -- but it startled me a little, and as soon as the ice was clear, I cut off the defroster. I checked again a little later, and the rate of energy use was still quite elevated. Why would that be? I quickly figured out how my plan for Supercharger stops would need to be adjusted to accommodate the energy use, and carried on.

It wasn't until a few days later that I realized what (I think) was the reason for the dramatically decreased range: a coat of bumpy ice over the body's surface not only added some weight, it also messed up the car's aerodynamics. The sloppy road surface contributed too, I'm sure, but it really seems to me that the drag issue must have been primary.

The rest of the trip was smooth as can be. I put over 3000 miles on the odometer in a week.

But the conclusion I took from the ice storm incident was that the Model 3 relies on the low, low drag coefficient quite a bit to achieve the range and efficiency the car has.

Okay, discuss amongst yourselves.

It wasn’t ice on the car. The layer of ice on the car is still the same shape as the car. And a bumpy uneven surface is most of the time more aerodynamic (think dimples on a golf ball). What was your heat set on? My bet is most of the increased energy usage was the heat. And were the roads snow/ice covered? If so that increases utilization. And I’m in Alaska, the heat is a huge drain. I’ve averaged over 400 Wh/mi since November.

 

[Spudford]

It wasn’t ice on the car. The layer of ice on the car is still the same shape as the car. And a bumpy uneven surface is most of the time more aerodynamic (think dimples on a golf ball). What was your heat set on? My bet is most of the increased energy usage was the heat. And were the roads snow/ice covered? If so that increases utilization. And I’m in Alaska, the heat is a huge drain. I’ve averaged over 400 Wh/mi since November.

You may be right, coleAK, and as an Alaskan you have tons more experience with winter driving than me, obviously. I hadn't driven in winter conditions for 15 years until then. But the circumstantial evidence still impresses me:

• I turned the heat completely off, and the energy use remained high.
• It stayed high when the road surface was clear and dry.
• When the ice crust cleared from the car, the energy use dropped back toward normal.
• But mostly, I noticed that the high energy use rate was happening at highway speeds, not at street speeds.

 

[Saghost]

It wasn’t ice on the car. The layer of ice on the car is still the same shape as the car. And a bumpy uneven surface is most of the time more aerodynamic (think dimples on a golf ball). What was your heat set on? My bet is most of the increased energy usage was the heat. And were the roads snow/ice covered? If so that increases utilization. And I’m in Alaska, the heat is a huge drain. I’ve averaged over 400 Wh/mi since November.

It's complicated. Bumps out into the windstream do add substantial drag, at least until the boundary layer gets thick enough to mask them.

That's why aircraft went to flush riveting in the 1930s, and why a relatively low speed aircraft like the V-22 has flush rivets over the first third and flat head rivets for the remainder.

By contrast, the golf ball example is divets into the surface, creating stagnant air pockets that have vortices in them, reducing the surface area exposed to the wind and showing boundary layer formation.

Then you get into the whole laminar versus turbulent flow discussion. It's complicated, and that's why they need professionals with fancy CFD software to design efficient cars.

But yes, lumps on the skin of the car will likely produce additional drag. Pushing through slush produces a surprising amount of drag, too. Both are likely contributors to the OP's experience.

 

[coleAK]

You may be right, coleAK, and as an Alaskan you have tons more experience with winter driving than me, obviously. I hadn't driven in winter conditions for 15 years until then. But the circumstantial evidence still impresses me:

• I turned the heat completely off, and the energy use remained high.
• It stayed high when the road surface was clear and dry.
• When the ice crust cleared from the car, the energy use dropped back toward normal.
• But mostly, I noticed that the high energy use rate was happening at highway speeds, not at street speeds.

That is crazy. How much ice was on the car? I was imagining just a thin layer. One thing I have noticed is the hood of the Tesla does freeze as there isn’t an ICE to heat it up. But I’ve only seen a glaze. But I figure it’s a wash considering the glass roof warms enough to defrost it.

What I’ve noticed in 4.5 winters with a Tesla in Alaska, 4 of them with an S and now 0.5 with an AWD LR 3. In terms of increased energy usage: #1 is the heat, #2 is increased rolling resistance and drivetrain demand on snow/ice/slush. #3 is cold soaked battery (when it’s < 0F). A distant #4 is everything else.